I heard this term on one of the many podcasts I listen to. Unfortunately I can't remember which podcast it was, so can't give credit. I'm almost sure it was one of the score or so that shelter under the Book Riot umbrella and that it was one of the women who said it. Eventually I will figure out how to add links to this blog so people can see what I'm listening to.
Interstitial reading describes the bits of reading we do in the spaces of our lives such as waiting in line, waiting in the doctor's office, red lights (which are unnaturally long in Tucson), in the bathroom, etc., etc. Now that I have an iPhone, I find that I can accomplish quite a lot of reading this way. I usually have several books going at once, but one generally has more pressure to finish it. I belong to two online and one real-life books clubs, so I'm always behind in reading at least one of them. By assigning the most pressing book to my interstitial reading time, I'm able to power through at a much faster rate.
All the other books I'm reading have to wait their turn in the queue on my Kindle. I read a chapter, then move on to the next. Takes me about a week to get through the entire list, so reading even a moderately-sized novel can take months. Obviously this won't do when I have to finish Crossing to Safety by the November 18 book club meeting. At my home. I have gone to book club meetings without finishing the book, but this book was my choice, so I feel obliged to finish it and come up with discussion questions before the meeting.
Hey, presto! Interstitial reading to the rescue. All those 5/10/15-minute periods add up to quite a lot of reading time. So many times during the day that I spend waiting. Now I can take advantage of them and put them to good use.